1. Field of the Invention
This invention relates to an apparatus for multiplexing signals on other amplitude-modulated signals.
2. Prior Art
Considering the transmission systems, for example, used for television broadcasting, more than twenty five years have passed since the color television broadcasting using the conventional National Television System Committee (NTSC) method started in 1960 in Japan. In this period, a variety of novel television systems were proposed in response to requests for a high definition image and the enhancement of the performance of television receivers. In addition, the programs of broadcasting have been changing from mere studio programs and live programs to those with high picture quality and realistic feeling such as cinema-size movies.
The conventional broadcasting operates on specifications such as 525 scanning lines, 2:1 interlace scanning, 4.2 MHz horizontal bandwidth of the luminance signal and 4:3 aspect ratio (refer, for example, to Pritchard, "US Color Television Fundamentals--A Review", IEEE Trans. Consumer Electron., vol. CE-23, pp.467-478, No. 1977). As described above, the present television broadcasting is restricted in its signal band by the standard, and hence it is difficult to add other mass information. In view of such a background, we proposed a television signal configuration method capable of multiplexing mass information in a specific band, while keeping compatibility with the conventional television system. (See Y. Yasumoto et al. "An Extended Definition Television System Using Quadrature Modulation of the Video Carrier with Inverse Nyquist Filter", IEEE Trans. Consumer Electronics, Vol. CE-33, pp. 173-180, Aug. 1987 and U.S. Application No. 070804 filed July 7, 1987).
Proposed was a system of amplitude-modulating, in the vestigial side band, a carrier P.sub.2, which is a multiple signal different from a main signal, that is, the amplitude-modulated television signal of the vestigial side band in the conventional television system, and has the identical frequency as that of a video carrier P.sub.1 and a phase differing by +90.degree. or -90.degree. from that of the video carrier P.sub.1, so as to remove the carrier P.sub.2 in the blanking period. This modulated signal is band limited by a filter having symmetrical characteristics with respect to the video carrier as compared to that of the video intermediate frequency filter in the conventional television receivers. This signal is added to the vestigial side band amplitude-modulated television signal, and the signals are then multiplexed. The television signal processing method on the receiver side of the television signal combined in the above way is explained below. In the conventional television receiver, the multiplex signal component forms double side bands. Accordingly, impairment by the multiplex signal of the conventional television receiver operation which executes a video signal synchronous detection does not occur theoretically.
A multiplex signal demodulation method in the receiver of the television signal combined in the above method is described next. The signal in the video intermediate frequency band which is an output of the tuner is band limited by a filter so that the video base band signal should form double side bands. When this band-limited signal is synchronously detected by a regenerated carrier of the multiplex signals, that is, a carrier I.sub.2 having an identical frequency to that of a video carrier I.sub.1 and differing in phase by +90.degree. or -90.degree. from that of the video carrier I.sub.1, only the multiplex signal components can be demodulated without any quadrature distortion. In the above publication of IEEE and in U.S. Application Ser. No. 175409 filed on Mar. 28, 1988, a multiplex signal processing apparatus was proposed which can transmit the television signals including image information with an aspect ratio of more than 4:3 by classifying the region whose aspect ratio is 4:3 from among the images having an aspect ratio of not less than 4:3 as main signals and the remaining regions as multiplex signals.
These proposals, however, have problems left unsolved from a viewpoint of impairment of the conventional television receivers. In particular, in the television receivers having a detection circuit using pseudo-synchronous detection or envelope detection, impairment to a certain extent is left in comparison with the television receivers having a detection circuit owing to the synchronous detection. It is moreover impossible to extend the transmission band considering the effective use of radio wave resources.